1. Field of the Invention
The present invention relates to an electronic article surveillance system and a marker for use therein; and more particularly, to a system comprising a miniature magnetomechanically resonant marker that enhances the sensitivity and reliability of the article surveillance system.
2. Description of the Prior Art
Attempts to protect articles of merchandise and the like against theft from retail stores have resulted in numerous technical arrangements. Among these, a tag or marker is secured to an article to be protected. The marker responds to an interrogation signal from transmitting apparatus situated proximate either an exit door of the premises to be protected, or an aisleway adjacent to the cashier or checkout station. A nearby receiving apparatus receives a signal produced by the marker in response to the interrogation signal. The presence of the response signal indicates that the marker has not been removed or deactivated by the cashier, and that the article bearing it may not have been paid for or properly checked out.
Several different types of markers have been disclosed in the literature, and are in use. In one type, the functional portion of the marker consists of either an antenna and diode or an antenna and capacitors forming a resonant circuit. When placed in an electromagnetic field transmitted by the interrogation apparatus, the antenna-diode marker generates harmonics of the interrogation frequency in the receiving antenna; the resonant circuit marker causes an increase in absorption of the transmitted signal so as to reduce the signal in the receiving coil. The detection of the harmonic or signal level change indicates the presence of the marker. With this type of system, the marker must be removed from the merchandise by the cashier. Failure to do so indicates that the merchandise has not been properly accounted for by the cashier. In addition, markers of these types typically are relatively expensive, making it economically desirable to reuse them.
A second type of marker consists of a first elongated element of high magnetic permeability ferromagnetic material disposed adjacent to at least a second element of ferromagnetic material having higher coercivity than the first element. When subjected to an interrogation frequency of electromagnetic radiation, the marker causes harmonics of the interrogation frequency to be developed in the receiving coil. The detection of such harmonics indicates the presence of the marker. Deactivation of the marker is accomplished by changing the state of magnetization of the second element. Thus, when the marker is exposed to a dc magnetic field, the state of magnetization in the second element changes and, depending upon the design of the marker being used, either the amplitude of the harmonics chosen for detection is significantly reduced, or the amplitude of the even numbered harmonics is significantly changed. Either of these changes can be readily detected in the receiving coil.
Ferromagnetic, harmonic-generating markers are smaller, contain fewer components and materials, and are easier to fabricate than resonant-circuit or antenna-diode markers. As a consequence, such a marker can be treated as a disposable item affixed to the article to be protected and subsequently disposed of by the customer. Such markers may be readily deactivated by the application of a dc magnetic field pulse triggered by the cashier. Hence, handling costs associated with the physical removal requirements of resonant-circuit and antenna-diode markers are avoided.
One of the problems with harmonic-generating ferromagnetic markers is the difficulty of detecting the marker signal at remote distances. The amplitude of the harmonics developed in the receiving antenna is much smaller than the amplitude of the interrogation signal, with the result that the range of detection of such markers is generally limited to aisle widths less than about three feet. Another problem with harmonic-generating ferromagnetic markers is the difficulty of distinguishing the marker signal from pseudo signals generated by nearby ferrous objects, including both items ordinarily found in the retail environment such as building structures, shopping carts, and display racks, and items routinely carried by shoppers, such as belt buckles, pens, hair clips, and the like. The merchant's fear of embarrassment and adverse legal consequences associated with false alarms triggered by such pseudo signals will be readily appreciated. Yet another problem with such ferromagnetic markers is their tendency to be deactivated or reactivated by conditions other than those imposed by components of the system. Thus, ferromagnetic markers can be deactivated purposely upon juxtaposition of a permanent magnet or reactivated inadvertently by magnetization loss in the second ferromagnetic element thereof. For these reasons, article surveillance systems have resulted in higher operating costs and lower detection sensitivity and operating reliability than are considered to be desirable.
Another type of marker is disclosed by U.S. Pat. No. 4,510,489 to Anderson et al. The marker comprises an elongated, ductile strip of magnetostrictive ferromagnetic material adapted to be magnetically biased and thereby armed to resonate mechanically at a frequency within the frequency band of the incident magnetic field. A hard ferromagnetic element, disposed adjacent to the strip of magnetostrictive material, is adapted, upon being magnetized, to arm the strip to resonate at that frequency. The strip of magnetostrictive material has a magnetomechanical coupling factor, k, greater than 0, given by the formula k=[1−(fr/fa)2]1/2, wherein fr and fa are the resonant and anti-resonant frequencies of the magnetostrictive element, respectively. In the presence of a biasing dc magnetic field the effective magnetic permeability of the marker for excitation by an applied ac electromagnetic field is strongly dependent on frequency. That is to say, the effective permeability of the marker is substantially different for excitation by an ac field having a frequency approximately equal to either the resonant or anti-resonant frequency than for excitation at other frequencies. A detecting means detects the change in coupling between the interrogating and receiving coils at the resonant and/or anti-resonant frequency, and distinguishes it from changes in coupling at other than those frequencies.
However, known resonant markers comprising magnetostrictive material and systems employing such markers, including those of the type disclosed by U.S. Pat. No. 4,510,489, have a number of characteristics that render them undesirable for certain applications. The markers are elongated and relatively large in size, especially in their longest direction. As a result, they are too large to be accommodated on some items of merchandise, including many for which protection is highly desirable because of their high value. A large marker is also relatively conspicuous when affixed externally to a merchandise item. In addition, the cost of the marker disclosed by U.S. Pat. No. 4,510,489 is necessarily governed by the size of the marker and the amount of the magnetic material that accordingly must be used.
There remains a need in the art for antipilferage systems employing markers that are small, light, and inexpensive to construct and reliably detected.